1. Field of the Invention
The present invention relates to manufacture of standard gas mixtures and, more particularly, to a process and an apparatus for producing standard gas mixtures for testing, calibrating, . . . highly sensitive analytical instruments such as an atmospheric-pressure ionization mass spectrometer (APIMS).
2. Description of the Background
A variety of gases are used in manufacturing semi-conductor devices such as LSIs. These gases contain impurities. The impurities have an adverse influence on the characteristics of the LSIs. Hence, it is demanded that the gases be as pure as possible. This demand grows stronger, along with the increase in the integration density of LSIs. To meet this demand, a high-accuracy and reliable analysis of gases is required.
The techniques commonly used for analyzing such gases for determining the impurity contents thereof are: gas chromatography, gas chromatography-mass spectroscopy, and Fourier transformed infrared spectroscopy. The detection limit of these techniques are, however, 1 to 10 ppb at best. In view of this, these analytical techniques cannot be said to determine the impurity content of the gases as sensitively as is required for example in the manufacture of LSIs.
Known as apparatus for producing standard gas mixtures for use in the various analytical instruments is a standard gas generator which dilutes a high-concentration gas mixture containing the impurity to be analyzed with a so-called zero gas in one or more stages, thereby generating a standard gas mixture under a predetermined pressure at a predetermined flow rate. This apparatus is very useful in producing standard gas mixture to be used to obtain calibration curves. However, it cannot be employed in the internal standard method or the standard addition method, wherein a known amount of a high-concentration standard gas is added directly to a sample gas. Nor can the apparatus be used to produce a standard gas mixture which contains multiple component gases.
Therefore, it has been demanded that a process and an apparatus be developed which can produce these various gas mixtures.
More generally, most of the following functions are required in the daily practice of gas analysis:
Introduce the gas to be analyzed in a controlled way into the analyzing device;
Add to the gas to be analyzed, calibrated amounts of gaseous species in order to apply the standard addition and internal standard generation methods;
Introduce a blanc or zero gas into the analyzing device;
Add to the zero gas calibrated amounts of gaseous species in order to generate controlled mixtures facilititating the calibration of the analyzing device;
Dilute in a controlled way the gas to be analyzed by the zero gas before introduction of the mixture into the analyzing device. This last point is particularly relevant for very polluted, or toxic and/or corrosive gases.